Speed ratings explained

When it comes to data transfer speeds, manufacturers often give an ‘x-factor’ rating. This is specified as a multiple of 150 kilobytes per second, the basic read speed of data from a compact disc. There are 1024 kilobytes in a megabyte, so a memory card with a 600x speed rating would have a transfer speed of 600x150/1024, or around about 88Mbps.

That’s not the whole story, however: the quoted figure is often for read speed, and the write speed can be significantly slower.

A clearer indication is given by a rating that simply states the transfer speeds, ideally for both reading and writing.

Decent sustained write speeds are essential for recording 1080p and 4K video. However, they can also be important for stills. So, if you’re capturing long sequences of images in Continuous drive mode at Raw quality settings, you can fill your camera’s memory buffer in seconds. Your camera will then slow down to a crawl or stop, preventing you from taking further shots before the buffer has cleared to the memory card. The difference between fast and slow cards can result in the wait being just a few seconds up to roughly a minute or so.

Before you just go and buy the fastest, most expensive cards around, there’s another factor to consider… The write speed quoted is the theoretical maximum: in practice, the capture, image processing and data writing for any camera can present a potential bottleneck if you’re not careful.